1. Field of the Invention
The invention, in general, relates to a gear shift apparatus for bicycles and, more particularly, to a gear shift apparatus of the kind utilizing a plurality of co-axially arranged sprockets of different sizes and a chain selectively movable into engagement with any one of the sprockets, such apparatus being conventionally known as a derailleur.
2. The Prior Art
Known derailleurs usually are provided with a shift member operating by way of a four-bar-linkage moving device, vice, the shift member being movable, during gear selection, against the bias of a spring by the wire of a Bowden cable. The Bowden cable transmits the force of an actuator operable by a bicyclist, to a derailleur arranged adjacent to the sprockets. The wire which is sectionally guided within a sleeve extends from the end of the sleeve connected to the derailleur, and in this area it is not protected from dirt or moisture. This is particularly disadvantageous because on a bicycle, this portion of the wire is positioned a small distance from the road surface, so that dirt may precipitate on the wire during drives over soft, muddy or dirty roads. The soiled portion of the wire can then no longer move un-impededly into the sleeve. As a result, the shifting mechanism may then be movable either by great force only or not at all. Moisture may also penetrate into the sleeve, especially at the actuator, and lead to corrosion of the wire. Another proven disadvantage is that the wire has to be installed such that neither sharp bends nor kinks can occur in the wire, since the friction between sleeve and wire, which is considerable in any case, would otherwise become intolerably large, and excessive force would have to be applied to actuate the gear shift. Also, the wire would be subjected to excessive wear and tear. Accordingly, the ways in which the Bowden wire can be installed between the handle bar and the derailleur is rather limited.
It is known to use a rotary handle as an actuator (German Offenlegungsschrift 3,719,421), in which the effect of excessive friction within the cable may be lessened by the pitch of a thread serving as the mover, albeit at a large rotational angle. The low thread pitch mandates a correspondingly large angle of rotation of the rotary handle to yield a large shifting force. The basic disadvantages of the cable may, therefore, not be solved by this arrangement. While in the derailleurs mentioned supra the detent device is usually positioned near the derailleur (German Patent 3,132,103), the rotary handle makes it possible to positioning the detent device close to the actuator (German Patent 3,719,421).
In a known bicycle gear shift for bicycles (German Offenlegungsschrift 4,241,521) a master cylinder is directly actuated by a manipulation lever, while a slave cylinder is connected by a cable to the derailleur for moving the chain guide sprocket. Hence, to be activated the piston of the slave cylinder is in one direction pushed against the bias of a spring connected to the cable. Return movement is imparted to a lever by a return spring arranged within the master cylinder. While this known gear shift in general avoids the disadvantages of a Bowden cable, it nevertheless retains the disadvantage of a wire near the derailleur. Furthermore, only the annular surface of the piston in the slave cylinder is acted upon, so that for the requisite shift force the piston must be of a relatively large diameter.
Hydraulic conduits have for a long time been known for transmitting power (French Patent 1,044,279 of 1951). Thus, it has been known to utilize a pivotable lever as an actuator which engages, by way of a pinion, a piston rod formed as a toothed rack of a master cylinder, while a piston rod in the slave cylinder, movable against the bias of a spring, directly engages the sprocket. This well-known hydraulic gear shift arrangement does, however, significantly protrude from the chain shifting mechanism and is thus rendered prone to damage and malfunctioning of the kind not readily repairable, such as, for instance, escaping hydraulic oil.
In yet another hydraulic derailleur (French Patent 2,154,018), a slave cylinder is rigidly connected to a bicycle frame. Such a structure suffers from the disadvantages of protruding significantly, so that it may easily be damaged, and of its slave cylinder having to transmit forces of the kind likely to result in leakage. In another embodiment of this known derailleur (FIG. 11), the slave cylinder is rigidly mounted on a component of the shift mechanism, and motion is transmitted to the chain guide of the shift mechanism by a bilaterally rotatably journal led intermediate member. In this arrangement the slave cylinder is not free of transverse forces. Moreover, because of the unfavorable geometry of the intermediate member the shifting force is depending upon the shift position. It is quite possible that the use of hydraulics in bicycle derailleurs has at best enjoyed limited success because of the described disadvantages.